That would depend on how you want Modric to control the game. If you want Modric to dictate play and play more of a Xavi role, then I would play a 4-3-3 and place him as an advanced playmaker-s. If, on the other hand, you want Modric further up the pitch in order to play the final through ball, then I would do a 4-2-3-1 and play him as a trequartista. However, in this setup, you wouldn't play a dm in between your 2 cbs. If its a must that you both play a single dm and you want Modric to play in a more advance position, then I would play a 4 man diamond midfield and keep Modric at AMC-trequartista. I don't like this formation as I'm partial to playing a 5 man mid, but it would allow you to maximize Modric's final pass ability and play a dm between your cbs.
If you go with the 4 man diamond mid, then I would like for other defensive minded mids with excellent d mental stats and stamina (especially work rate, positioning, teamwork and anticipation) to provide defensive cover against 5 man mids. This is especially true if you give Modric a free role. Another option would be playing a 3-2-2-1-2. You could retrain your DM as a cb and play him as a stopper (or as a libero if you want to get real creative) and then play Modric as the enganche behind your forwards. This would be great in a counter attack setup, but you should have a back up strategy when you play against elite dms as theyll mark him out of the gam.